The characteristics of a postulated cholinergic neural system involved in control of cerebral circulation will be investigated in rabbits by determining the following: 1. changes in cerebral oxygen consumption (CMRO2) and cerebral blood flow (CBF) produced by intravenous administration of physostigmine and atropine; 2. effects on local cerebral tissue pH, PO2 and blood flows induced by topical application of cholinomimetic and cholinolytic drugs; 3. effects of section of VIIth cranial nerve and of the cervical sympathetic on resting CBF, CMRO2, CBF changes associated with cortical desynchronization, and CBF changes induced by physostigmine; 4. ability of physostigmine to restore CO2 reactivity after precollicular decerebration; 5. effects of stimulation of brain stem sites on CBF, arterial pressure, EEG and CMRO2.